This invention relates to a multipath noise reducer, an audio output circuit including a multipath noise reducer, and a frequency-modulation (FM) radio receiver including a multipath noise reducer.
Radio receivers are afflicted by various types of electromagnetic noise. Radio broadcast receivers mounted in automobiles, for example, must contend with ignition noise and mirror noise, which are impulsive in character and are generally referred to as impulse noise. These so-called car radios also experience episodes of multipath noise due to reflection of radio waves from hills, high buildings, and other passing objects. Multipath noise occurs because the car radio antenna receives both a line-of-sight signal, coming directly from the transmitting antenna, and reflected signals, reflected from the passing objects. The reflected signals tend to be out of phase with the line-of-sight signal, causing the line-of-sight signal to be partly attenuated by the reflected signals. The resulting deterioration in quality of the audio output from a car radio is a familiar experience to automobile riders.
Various methods of reducing noise are known. In an FM stereo car radio, one method is to detect the strength of the electric field received at the antenna, and take noise countermeasures when the field is weak. One countermeasure is to reduce the degree of stereo separation, or to switch completely from stereo to monaural operation. This countermeasure will be referred to below as stereo separation control. Another countermeasure is to attenuate or xe2x80x9ccutxe2x80x9d high-frequency components in the demodulated signal. This countermeasure will be referred to below as high-cut control. Both of these countermeasures improve the signal-to-noise (S/N) ratio during intervals when the electric field received at the antenna is weak.
To reduce impulse noise, car radios may also include an impulse noise reducer that detects the onset of impulse noise and generates a gate signal having a predetermined length sufficient to cover the expected duration of the impulse noise. When the gate signal is active, the signal output by the car radio is held constant, effectively suppressing the noise.
The gate pulse used in this type of impulse noise reducer is too short to mask multipath noise, the duration of which is typically much longer than the duration of impulse noise. The gate pulse could be lengthened to cover multipath noise intervals, but a long gate pulse would noticeably distort the audio output signal. Furthermore, the long gate pulse would be triggered by each short occurrence of impulse noise, resulting in much needless audio distortion during times when no noise was present.
Another problem is that although the effects of multipath noise vary depending on signal reception conditions and the audio signal level, the gate pulse width is conventionally the same for all reception conditions and audio signal levels. Accordingly, regardless of how the gate pulse width is set, it will sometimes be too long, causing needless audio distortion, and will sometimes be too short, so that multipath noise is inadequately reduced.
Further details of these problems will be given in the detailed description of the invention.
An object of this invention is to reduce multipath noise adequately, with minimal output distortion.
The invented multipath noise reducer includes a signal state determiner determining a state of an input signal, a threshold generator generating a threshold value responsive to the resulting state information, a high-frequency signal extractor detecting high-frequency components of the input signal, a comparator unit comparing the resulting high-frequency signal with the threshold value, thereby generating a multipath noise detection signal, and a correction unit modifying the input signal responsive to the multipath noise detection signal and the state information.
By comparing the high-frequency signal with a threshold value, the invented multipath noise reducer is able detect and remove individual multipath noise spikes, thereby removing bursts of multipath noise without distorting other parts of the input signal.
By determining the threshold value adaptively, on the basis of the state information, and by modifying the input signal adaptively, again on the basis of the state information, the invented multipath noise reducer is able to reduce multipath noise adequately under all signal conditions, without unnecessary distortion.
The multipath noise reducer preferably also includes an input smoothing unit that smoothes the input signal. The smoothed input signal is used when the input signal is modified, enabling the correction unit to reduce distortion in the corrected signal still further.
In this case, the correction unit preferably includes a gate generator that generates a gate signal by expanding pulses in the multipath noise detection signal by an amount depending on the state information, and a replacement unit. The replacement unit latches the smoothed input signal during each expanded pulse in the gate signal, and replaces the input signal with the latched value for the duration of the expanded pulse. The length of the gate pulse is thereby tailored to signal conditions, and replacement of the input signal with possibly distorted values is avoided.
The gate generator preferably expands the gate pulses by increasing amounts as the received field strength of the input signal decreases, so that as the effects of multipath noise worsen, more of the multipath noise is removed.
The gate generator also preferably expands the gate pulses by increasing amounts as the audio signal level decreases, so that as multipath noise becomes more noticeable, more of the multipath noise is removed.
The high-frequency signal extractor preferably includes a high-pass filter and an absolute-value calculation unit, which together generate a high-frequency signal suitable for comparison with a threshold value.
The multipath high-pass filter preferably receives input from the absolute-value calculation unit, an arrangement that tends to shorten the intervals in which multipath noise is detected so that they match the actual multipath noise intervals more closely.
The threshold generator preferably includes a high-frequency smoothing unit that smoothes the high-frequency signal, and an adaptive limiting unit that limits the smoothed high-frequency signal according to the state information. The threshold value can thereby be kept from becoming too large during episodes of multipath noise.
The threshold generator may also include an amplitude limiter that limits variations of the high-frequency signal before the high-frequency signal is smoothed, so that the threshold value can be kept from becoming too large without the need for a long smoothing interval.
The adaptive limiting unit preferably includes a parameter adjustment unit that selects a comparison value and a limit value responsive to the state information, and a limiting unit that reduces the high-frequency signal to the limit value when the high-frequency signal exceeds the comparison value. The threshold value can thereby be lowered during episodes of multipath noise, so as to be sure of detecting all of the multipath noise.
The parameter adjustment unit preferably increases the comparison value as the received field strength of the input signal decreases, to avoid reducing the threshold value when multipath noise is absent.
The invention also provides an audio output circuit including the invented multipath noise reducer.
The invention furthermore provides an FM receiver including both the invented multipath noise reducer and an impulse noise reducer, the impulse noise reducer removing residual impulse noise from the corrected signal output by the multipath noise reducer.
The invention moreover provides a method of reducing multipath noise, essentially as described above. The invented method is useful when the invention is practiced using digital signal-processing circuitry.